1. Field of the Invention
This invention relates to method and apparatus for forming fibrous webs from a gaseous suspension of fibers, and more specifically to an improvement in such method and apparatus for achieving high speed, high quality formation of nonwoven fibrous webs.
2. Description of the Prior Art
Air-laid fibrous webs are becoming extremely popular for use in disposable products, such as disposable diapers, sanitary napkins, cosmetic pads, industrial towels, household towels, tissues and the like. These products are generally intended for single, or limited use applications, and accordingly, it is important that they be capable of fabrication in an economical manner. One of the most significant problems encountered in the fabrication of air-laid fibrous webs has been to achieve high speed, high quality production so as to minimize manufacturing costs.
A commonly employed apparatus for forming an air-laid fibrous web includes a fiberizing roll for separating fibers from a fibrous feed mat and entraining the fibers in air to form an air suspension of said fibers. The air suspension of fibers is directed through an upstream end of a formation duct toward a moving foraminous forming surface which traverses a downstream open end of said duct. The air from the suspension is directed through the foraminous forming surface, and the fibers from the suspension are condensed upon the forming surface to form the air-laid fibrous web. A partial vacuum is established behind the forming surface to aid in directing the air suspension of fibers toward it. Quite often the formed webs are subjected to conventional post-treatment operations; such as embossing, adhesive bonding, and the like; to impart desired properties to the webs.
The above-described apparatus has a web formation area on the foraminous forming surface bound by the walls of the formation duct. It is known that a large formation area is desirable to minimize the pressure drop through the foraminous forming surface. It is suggested in U.S. Pat. No. 2,703,441, issued to Langdon et al., to increase the formation area on a forming surface by inclining the forming surface diagonally to the central axis of a formation duct, and by establishing a relatively large divergence angle between front and back inner surfaces of said formation duct. Applicants have found that such an arrangement can not be employed to achieve high quality, air-laid web formation at high production speeds, i.e. over 200 feet per minute. Specifically, the geometry of the Langdon et. al. device creates air-flow instabilities in the gaseous suspension, and at high production speeds these instabilities create fiber clumps and excessive basis weight variations throughout the web. In fact, in some instances holes may actually be formed in the web.
British Patent Specification No. 883,139, discloses an apparatus for manufacturing a textile fiber web. The apparatus includes a fiberizing roll for separating fibers from a feed mat and directing the fibers in a gaseous suspension through a formation duct to a foraminous, cylindrical forming surface upon which the web is formed. In that apparatus it is required that the machine casing conform in shape to the periphery of the fiberizing roll so that the duct area between the stripping point on the fiberizing roll and the cylindrical condenser be substantially uniform or gradually increasing. Although such an arrangement allegedly will minimize the creation of undesirable eddies, or pressure variations in the air stream, the British specification is silent as to other parameters which applicants have found to be extremely important to their invention. Specifically, the British specification does not disclose any criticality in the particular angle of incidence between the formation duct and the forming surface, nor any specific critical relationship between the angle of incidence and the included angle between front and back inner surfaces of the formation duct. These parameters have been found, in the instant invention, to be critical to the high speed, high quality formation of air-laid fibrous webs.
Moreover, the arrangement of the air-conveying ducts in the apparatus disclosed in the British specification precludes high speed operation to produce high quality fibrous webs. Specifically, at high production speeds a high velocity air flow must be directed into the upstream end of the fiberizing zone to establish the fiber path depicted in FIG. 1. Since the opening into the upstream end of the fiberizing zone is large, a large volume of air is required to establish this high velocity flow. Accordingly, a high vacuum level would also have to be established behind the forming surface to remove the large volume of air directed into the fiberizing zone. The use of a high vacuum level can cause the fibers deposited on the forming surface to become excessively stapled to the forming surface; thereby causing the web to become damaged as it is removed therefrom.
Several different embodiments of air-laydown apparatus are disclosed in U.S. Pat. No. 3,797,074, issued to Zafiroglu. In all embodiments a formation duct is disposed between a fiberizing roll and a foraminous surface upon which a web of staple-length fibers is formed. The formation duct is oriented to the foraminous surface at either about 90.degree. or at 45.degree., and absolutely no criticality is alleged, or suggested, as to the desired orientation of the formation duct with the foraminous surface. In fact, the orientations disclosed in the Zafiroglu patent are not within the critical range of applicant's invention.